Frequency shift pulse time modulation



c. H. McSHAN 2,451,347

I /4 g g EEOUTPUT Oct. 12, 1948.

Filed Jan. 17, 1947 CRYSTAL osclLLAmle a name 'I" 4 FIGURE 1 CRYSTAL /0- OSCILLATOR MOD PULSE TIME REACTANCE VARIABLE R F MODULATED TUBE OSCILLATOR 'NPUT Mz/LT/V/BPAm/e 15 LOCK/N f, 0M f cm s TAL 38 11 OSCILLATOR FREQ. Q

HGUREZ I C/arence Hum'er M 5han INVENTOR.

1. ATTORNEYS- Oct. 12, 1948. c, McSHAN 2,451,347

FREQUENCY SHIFT PULSE TIME MODULATION Filed Jan. 1.7, 1947 2 Sheets-Sheet 2 OSCILLA T012 OSCILLATOR /o E FREQfi mm cuecwr 15 PULSE TIME MODULA TED MUL T/I/IBRA TOR FREQfiOEff OUTPUT 11 CRYSTAL OSCILLATOR FREQ. F

CRYSTAL OSCILLATOR I0- FREQ. F,

CRYSTAL OSCILLATOR 11- FREQ. F

PULSE TIME MODULA TED I MuLT/v/EmTv/e MODULATION CHA NNEU'I PULSE OUTPUT MODULATION FIGURE 6 CHA NNEU'Z Clarence Humer M Shan IN V EN TOR.

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ATTORNE Y5 Patented Oct. 12, 1948 UNITED STATES PATENT OFFICE FREQUENCY SHIFT PULSE TIME MODULATION Claims.

The present invention relates to modulation systems for use with radio frequency carrier signals, and more particularly to new and improved pulse time modulation systems which are simple in construction and highly effective in operation.

Modulation systems have been devised that are characterized by low noise background reception. Examples of such systems are the commonly known forms of frequency modulation, including the rmstrong phaseshift method, the center frequency control system using a reaction tube and variable oscillator, and. more recently, the piasitron method, which is an improvement over the phaseshift method. It has also been proposed to generate pulses, thepulse width bein a function of the amplitude of a modulating signal, and to use the width modulated pulses to nQdulate, a conventional frequency modulated transmitter. While such systems have. been used efiectively, they are relatively complex and are not particularly suitable for use in low cost crystal controlled transmitters, where minimum space requirement and low power consumption are of prime. importance.

The principal objectv of the; present invention, accordingly, is to provide new and; improved pulse time modulation systems which are free from the above noted defects of the prior art, and are simple, construction and highly effective in operation,

A, further object of the invention is to produce new and improved pulse: time. modulation systems of the above. character which provide signals that may be received with conyentionel frequency.

modulation eceivin qu pment us n a i criminator and. a limiter, for example.

Still another objectof the invention is to provide new and improved pulse time modulation systems of the, above, character which provide low noise background reception and embodyrelatively simple. circuits possessing excellent stability.

Another object of the invention is to provide a combined pulse. time modulated and pulse frequency modulated system which affords at least. two transmission channels with av mini-mum of equipment aril lrwith highly satisfactory operating haracter stics...

The, objects of the invention are attained by providing a. pulse t me, modu ation sy e in which radio. frequency carrierpulses of different frequencies are. alternately. transmitted in such fashion. that substantially no time, delay exists between suc es ive puls s f d fferen f i Th puls widt s of a h onse utive. p ir of ar er ou ses fdifierent fr quencies a e. m

lated in accordance with the amplitude of a v modulating signal in complementary fashion such that a decrease'in the width of one pulse of one frequency is accompanied by an increase in the width of the next pulse of different frequency, while the total pulse width for any two consecutive pulses is maintained substantiallyconstant. The relationbetween the width of each pulse and. the instantaneous modulation amplitude of the intelligence which is to be transmitted by the modulation system is preferably made substantially linear.

In one embodiment of the invention, a pair of crystal oscillators of different frequencies provide outputs to a single resonant circuit capable of: transmitting both frequencies. The outputs fromthe two crystal oscillators are controlled by any suitable pulsing meanssuch as a multivibra-tor,

for example, such that the outputs of the twocrystal oscillators are alternately supplied to the resonant circuit. The multivibrator is controlled by the modulation that is to be transmitted in such fashion that the pulse widths of successive half-cycles of the multivibrator output are modulated linearly with respect to the modulation input.

In accordance with another embodiment of the invention, a variable oscillator is adapted'to lock' in. alternately with the two crystal oscillators of different frequencies. The locking-in of the variable oscillator may be controlled in any suitable manner as by means of a reactance tube, for

example, pulse time modulated by means of a multivibrator in the manner outlined above, or by other suitable means. U

Instead of using a variable reactance tube to control a variable oscillator, pulses from the multivibrator or other like means may beused to control the alternate insertion and removal of additional reactance in the oscillator tank circuit, to produce alternate carrier pulses of different frequencies in accordance with the invention. Modulation of the lock-in oscillator frequency by the suppressed frequency (the frequency not being transmitted at the. moment). may be eliminated by limiting the. lock-in oscillator output by any one. of several well known means.-, I

The invention also contemplates modulating;

the repetition rate of. the controi. pulses in. accordance with a modulating. signal.

tional type.

With. this con- 1 struction; twotransmission channels are pro vided; one using pulse time modulation 'and the other using frequency modulation of. the conven- 3 It will be apparent from the foregoing that the novel pulse time modulation system of the present invention affords essentially the same advantages as conventional frequency modulation equipment with less equipment and with relatively simple circuits possessing excellent stability. Further, it enables the use of oscillators which may employ crystal control. In addition, the pulse timemodulated signals that are produced are capable of being received on standard frequency modulation receivers now in general use, and the system is characterized by low noise background reception.

Additional objects and advantages of the invention will be apparent from the following detailed description of several representative embodiments, taken in conjunction with the accompanying drawings in which:

Figure 1 is a schematic diagram of a pulse time modulation system constructed according to the present invention;

Figure 2 is a modification of the modulation system shown in Figure 1 in which a variable oscillator is employed which is adapted to lock in with either of two oscillators operating on different frequencies;

Figure 3 is a schematic diagram of another embodiment of the invention in which modulation of the pulse time is produced by controlling the insertion and removal of an additional reactance in a tank circuit;

Figure 4 illustrates a further embodiment of the invention utilizing rectifying means; and

Figure 5 is a schematic diagram of still another embodiment of the invention which affords two separate and independent transmission channels.

In the'system' shown in Figure 1 of the drawings, two radio frequency signals are provided by the crystal controlled oscillators Ill and II, for example, whose frequencies are f1 and f2, respectively. The outputs of the oscillators I and H are supplied to the-grids of a pair of electronic I tubes I2 and I3, respectively, which may be of any suitable type containing at least three elements (grid, plate and filament). The outputs of each of the tubes 'I2 and I3, respectively, are

supplied to a common resonant circuit I4 whose, band width is made sufficiently Wide to pass signals of the twofrequencies f1 and f2. Signals of frequences f1 and f2 are alternately fed to the resonant circuit I4 in any suitable manner, as by alternately-blocking the tubes I2 and I3, for example.-

Blocking of the tubes I2 and I3 is preferably accomplished by alternately driving their cathodes positive with respect to their grids until plate current ceases to flow. The blocking voltages for the tubes I2 and I3 may be supplied from a conventional type multivibrator I5, for example. The multivibrator I may be of the usual type whose basic principle of operation is known to those skilled in the art, and it will not be described in detail herein. The biasing volt age pulses from the multivibrator I5 are taken off at the'cathode coupling resistors I6 and I1, respectively, as shown in Figure l.

The modulation voltage representing the intelligence to be transmitted is impressed upon the inputterminals I8 of a transformer I9, the secondary winding '20 of which has a center tap 2|. The center tap 2| is connected to the posi- 4 triode tube 26. The terminals 21 and '28 of the secondary winding 20 of the transformer I9 are connected through the resistors 29 and 30, respectively, to the control grids 3| and 32, respectively, of the tube 26. A condenser 33 is connected to the plate 25 and grid 3| of the tube 26, and a condenser 36 is connected to the plate 24 and the grid 32 of the tube 26.

With this construction, the modula-ticginivoltage applied'to the multivibrator I5 isi'n series with the grid return to the positive terminal of the plate supply :and is out of phase in each leg of the multivibrator. Hence, the condensers 33 and 34, respectively, will have their voltage decay times alternately increased or decreased, thereby increasing or decreasing the holding or conducting time of the respective triodes in the multivibrator tube 26, depending upon the polarity of the modulating voltage. This effectively gives a time modulated pulse voltage which is used to transmit alternately the-two radio frequency signals f1 and f2 from the oscillators Ill and I I, respectively, as will be described in:

greater detail hereinafterx Preferably, the relation between the pulse time of the pulses supplied by the multivibrator I5 and the instantaneous modulation amplitude of the discharge characteristic curve. This maybe accomplished, for example, by supplying a positive a voltage to the grid return at the center tap'ZI of the transformer secondary 20, whose value is at least equal to the plate voltage supplied to the tubes in the multivibrator I5.

When the modulating signal applied to the multivibrator I5 is zero, pulses of frequencies fl and f2 will be alternately transmitted from the output of the resonant circuit I4, the duration of each pulse being approximately half the total duration for the two pulses under optimum conditions. quency fa will remain substantially constant and it should preferably be made about three times the highest audio frequency in the intelligence which is to be transmitted.

Generally' speaking, the pulse frequency In should be made very much smaller than either of the radio frequencies, f1 or is. Accordingly, low-pass filters 35 and 36 maybe used between the cathode resistors I6 and I1, respectively, of the multivibrator I5 and the blocking stages comprising the tubes I2 and I3, respectively. Preferably, the upper cut-off frequency for the low pass filters should be at leastten times the pulse frequency ,fc in order 'to insure that the output.

pulses will have good, square wave form. The purpose of the filters 35 and 36 is to prevent signals of frequencies f1 or f2 from entering the circuit of the multivibrator I5. They also tend to reduce degeneration in the blocking stages comconventional type frequency modulation receiver using a discriminator and limiter, the operation of which elements is well known in the art. By'

tuning the receiver'to locate signals of frequencies f1 and f2 above and below the resonant frequency of the discriminator, respectively, alternate pulses of frequencies, f1 and f2 will produce pulses or alternate polarity in the output'of the discrimi- With properadjustment, the pulse frenator. When these time modulated pulses are passed through a low-pass filter, they are converted back to substantially the original modulating voltage. A low-pass filter exists in most frequency modulation receivers for the deemphasis of the high frequencies and it may be used satisfactorily for this purpose. Better results may be obtained if a filter having a sharp cut-off at the highest desired frequency is used, thus preventing the frequencies near the high end of the spectrum fromxbeing attenuated, as occurs with the conventional R. C. filter used for deemphasis purposes in standard frequency modulated receivers.

Since pulses of frequencies f1 and f2 follow each other immediately, the result is a substantially constant transmission, which prevents the occurrence of dead spots through which noise can enter the receiver. When either pulses of frequencies ji or ]z are completely out off, the threshold of improvement is lessened, or, the necessary signal strength to over-ride the noise is considerably increased. In addition, the volume of the demodulated signal for a given limiter level is approximately half that obtained with both pulses. Therefore, the use of two crystal controlled oscillators I0 and H to obtain a continuous transmission is justified, especially when considerable power is used, as the increase in allaround eificiency of transmission and reception obviously offsets the additional cost of a lowpower crystal oscillator and blocking circuit.

The Federal Communications Commission defines the carrier frequency f0 of an FM system as being the mean or average frequency transmitted. For this system,

Also, their regulations set the maximum band width at 150 kc. or :75 kc., either side of the carrier frequency, f0, so that the maximum difference frequency between the two oscillators must not exceed 150 kc.

Tests of the novel pulse time modulation system comprising the present invention have been made using a carrier average frequency f0 of 4.3 me. in order to feed the signal directly into a standard 4.3 me. I. F. amplifier with the usual limiter and discriminator. The effective band width of the receiver was approximately 200 kc. The multivibrator frequency, it was set at kc. The two oscillators were set at :75 kc. of 4.3 mc. Observation of the discriminator output on the oscilloscope to determine the frequency response and wave form showed substantially constant output between 90 and 5,000 0. P. S. with less than 5% distortion.

In the modification shown in Figure 2 of the drawings, the outputs of the two oscillators l0 and I l of frequency f1 and f2, respectively, are coupled loosely into the tank circuit of a variable oscillator 31. The oscillator 3'! is adapted to be controlled by means of a conventional type reactance tube 38 in such fashion as to lock-in alternately with one or the other of the two crystal controlled oscillators I!) or II when the reactance tube 38 is pulsed by the pulse time modulated multivibrator I5.

A further modification of the system shown in Figure 2 is illustrated in Figure 3 in which the resonant frequency of the tank circuit. 39 isv adapted to be changed by means of aby-pass or blocking condenser 40, a crystal rectifier M, and a variable condenser 42, all connected in series and in parallel with the tank circuit 39 of the variable oscillator 31. The chokes 43 and 44 serve to isolate the radio frequency from the circuit of the multivibrator [5.

As pulses are received from the multivibrator IS, the voltage across the diode 41. will be alternated in polarity and will preferably be at least equal to the peak R. F. frequency voltage impressed on the diode M from the tank circuit 39 when in operation. When the diode 4| is conducting, its impedance may be about 30 ohms, for example, and it effectively connects the condensers and 42 in series across the tank circuit 39 of the oscillator 31. When the diode 4| is not conducting, its impedance is very high so that it has relatively little effect upon the tank circuit 39. In a typical circuit, the capacitance between the elements of the diode 28 may be approximately two micro-microfarads. A frequency of 3000 mc. must be reached before the capacitive reactance equals 30 ohms, so that a substantial change in frequency may be had for much lower frequencies, where this circuit will generally be used.

The circuit shown in Figure 3 is independent of voltages insofar as frequency is concerned, which is an advantage over the reactance tube used in the embodiment illustrated in Figure 2, and, in addition, requires fewer components. It will be understood that this circuit may be used in a wide variety of applications for changing the impedance in any circuit by means of a reversal of a D. C. voltage.

Figure 4 illustrates still another embodiment of the invention in which rectifying means controlled by the multivibrator l5 employ d f0" impressing the outputs of the oscillators l0 and II selectively upon a tank circuit 45. In this embodiment, a pair of rectifiers such as, for example, the diodes 46 and 41, respectively, are alternately biased b the pulse modulated voltage entering the conductors 48 and 49 in such a, manner as to'render one conducting and to bias the other substantially below cutoff. The condensers 50 and 5! serve to block off the passage of D. C. to the crystal oscillators l0 and H, respectively, and the chokes 52 and 53 are employed to decouple the radio frequency from the common connecting lead 43. which is lay-passed to ground by the condenser 52a.

Any capacitive coupling existing across the diodes 46 and 41 may be neutralized in any suitable manner. The output may be taken off in convenient fashion, such as, for example, by means of a coupling coil 55. The condenser 54 serves to by-pass the tank circuit to ground, as shown in Figure 4. The by-pass condensers 52a, and 54 should preferably be made small enough to allow the time modulated pulses from the multi vibrator I5 topass with little distortion, while effectively by-passing the two radio frequency signals f1 and f2 to ground.

Pulse width modulation has the advantage of not being affected by non-linear distortion in the transmission path. Moreover, constant amplitude pulse modulation is the only type that can be applied in the case of certain types of VHF generators. For example, two magnetrons, each designed to operate at different frequencies, may be alternately pulsed by means of pulse width modulation to obtain higher power transmission in the VHF range.

In the system described above, a symmetrically modulated multivibrator circuit has been used for the purpose of maintaining a substantially constant vibrator frequency. With this construction, a pulse center is furnished which is constant in time repetition andwhose sides vary in accordance with the modulation voltage. It will be understood that one side only maybe made to vary in time, in which case the front edge of the pulse will be held stationary with respect to time. This form of pulse modulation is desirable when the multivibrator frequency is synchronized with a stabilizing frequency.

In the embodiment illustrated. in Figure 5 of the drawings, a system is illustrated by means of which both pulse time modulated and frequency modulated signals may be transmitted. Referring to Figure 5, the multivibrator l 5 is the same as in Figure 1 except that a second modulating transformer 56 is inserted between the mid tap 2| and the resistors 22 and 23. In this case, one modulating signal is supplied to the terminals [8 of the transformer I9, while a second modulating signal is supplied to the input terminals 51 of the transformer 56.

It has been found that the modulating voltage input supplied to the transformer 56 will vary the frequency rate of the multivibrator [5 without materiall upsetting the pulse time ratio as governed by the channel through the transformer Ill. The channel through the transformer [9, on the other hand, does not materially alter the pulse frequency in comparison with that obtained by'the channel through the transformer 56, so that two separate and independent channels of communication are produced whichare characterized by a minimum of cross-talk.

The output of the system shown in Figure 5, comprisingthe pulse time modulated signal and the pulse frequency modulated signal, may be received by a conventional type frequency modulation receiver. The frequency modulated signal can be readily demodulated since the regular pulse time modulation as supplied at the discriminator and low-pass filter outpiut of the receiver appears to be substantially free from the effects of pulse frequency modulation. By adding a second discriminator to take the multivibrator frequency ,f(: from the regular discriminator of the receiver, a second channel is provided whose response is substantially independent of the pulse time modulation. 1

This is true because pulse time modulation is, in effect, a form of phase modulation, the phase shift being substantially smaller than the frequency shift provided in the second channel, while, on the other hand, frequency modulation of the pulse rate is practically non-responsive to the first discriminator output due to the integrating eifect of the low-pass filter in the output of the receiver. cross-talk may be held low enough not to be objectionable for telephone and similar services. It is also possible that cross-talk between channels may be balanced out with suitable networks in the audio circuit. The second. frequency modulated channel enhances the general value of the system according to the invention with very little additional equipment.

In general, about 30% modulation of the pulse width is practical in pulse time modulation sys- With proper design proportions,

retaininga'rectangular shape. It can be shown 7 that the general requirement for bandwidth is: Band Width=10fc+Af 3 fa+Af where is is the pulse rate, fa is the highest audio frequency to be transmitted, and M is the frequency separationof the two R. F. carriers of frequencies f1 and f2. It is assumed here that fc=3fe for reasonably low distortion. This equation gives an approximate and conservative value of the band width required.

It will be understood that the several circuits described in detail above are intended merely to be illustrative of the invention and are capable of modification in a number of respects within the scope of the invention. For example, any other suitable pulse generating device may be used instead of the multivibrator shown." In fact, any device capable of pulsing two frequencies alternately while maintaining the total period substantially constant, and varying the time relationship of pulses of frequenciesfi and f2 in accordance with the amplitude of the modulation voltage, is capable of providing frequency shift pulse time modulation. Harmonic gener ators are occasionally used in transmitters employed for pulse time molulation, in which case the blocking stage may be used advantageously as a harmonic generator, provided that it comprises at least a triode. It. will be further understood that conventiona means may be employed in the foregoing systems to isolate the oscillator from the blocking tube to prevent adverse reaction on the oscilla tor stability. Additional isolation stages may be inserted, and other refinements common to the art may be made within the scope of the invention. 5

The novel modulating systems disclosed herein have wide application in the electronic art For example, they may be effectively used as audio or video channels, automatic volume controls, ringing circuits, synchronizing channels, control channels, automatic tone controls, and two dimensional sound systems. Other applications will be apparent to those skilled in the art.

While several representative embodiments have been described in detail herein, the inven-. tion is not to be limited thereto, but is susceptible of numerous changes in form and detail within the scope of the appended claims.

I claim:

1. In a radio frequency system, the steps of alternately producing radio frequency carrier signals of two different frequencies in the form of consecutive pulses of given time duration, and modulating said carrier pulses in accordance with a modulating signal.

2. In a radio frequency system, the steps of alternately producing radio frequency carrier signals of two different frequencies in the form of consecutive pulses of'given time duration, and modulating the time durations of the alternate pulses in accordance with a modulating signal While maintaining the total time duration of'each consecutive pair of pulsessubstantially constant. 3. In a radio frequencysystem, the steps ofalternately producing radio frequency e carrier pulses of two different frequencies, modulating the frequency of said carrier pulses inaccordance with one modulating signal 'while'maintaininga substantially constant pulse time proportion between each consecutive pair of pulses, and modulating the widths of the alternate pulses in accordance with another modulating signal while maintaining a, substantially constant pulse frequency.

4. In a radio frequency system, the steps of alternately producing radio frequency carrier pulses of two different frequencies, modulating the widths of the alternate pulses in accordance with the amplitude-of a modulating signal, such that opposite, complementary changes in pulse width are produced in each consecutive pair of pulses, whereby the total 'pulse width for each consecutive pair of pulses is maintained substantially constant, and transmitting said modulated pulses.

5. In a radio frequency system, the steps of generating two radio frequency signals of different frequencies, alternately blocking said two radio frequency signals in such fashion as to produce alternate radio frequency carrier pulses of said different frequencies, modulating the duration of said alternate pulses in accordance with the amplitude of a modulating signal while maintaining the total duration of each consecutive pair of pulses substantially constant, and transmitting said modulated pulses.

6. In a radio frequency system, the steps of alternately producing radio frequency carrier pulses of two different frequencies, and impressing at least two different types of modulation on said carrier pulses in accordance with corresponding modulating signals.

7. In a radio frequency transmission system, the steps of alternately producing radio frequency carrier pulses of two different frequencies, modulating the frequency of said carrier pulses in accordance with the amplitude of one modulating signal while maintaining a substantially constant pulse time proportion between each consecutive pair of pulses, modulating the widths of the alternate carrier pulses in accordance with the amplitude of a second modulating signal, such that opposite and complementary changes in pulse width are produced in each consecutive pair of pulses, while maintaining a substantially constant pulse frequency, and transmitting said modulated pulses.

8. In a radio frequency system, the combination of means for alternately producing radio frequency carrier signals of different frequencies in the form of consecutive pulses of given time duration, and means for modifying the time durations of said carrier pulses in accordance with a modulating signal, while maintaining the total time duration of each consecutive pair of pulses substantially constant.

9. In a radio frequency system, the combination of means for generating two radio frequency carrier signals of different frequencies, means for alternately suppressing first one and then the other of said carriers to produce alternate carrier pulses of different frequencies, means for changing the widths of alternate of said carrier pulses in accordance with the amplitude of a modulating signal, and means for changing the widths of the intervening carrier pulses between said alternate pulses in inverse relation to the amplitude of said modulating signal, whereby opposite, complementary changes in pulse width are produced in each consecutive pair of pulses, and the total pulse width of each consecutive pair of pulses is maintained substantially constant.

10. In a radio frequency system, the combination of means for generating two radio frequency carrier signals of different frequencies, an electrical output circuit connected to receive inputs from said generating-means, means for rendering said electrical outputcircuit'; alternately responsive to said'carrier signalswhereby' alternate carrier pulses 'of' said differentfrequencies" are produced, and means for controlling said last named means in accordance witha modulating signal such that the widthsof said pulses are modulated in accordance with said modulating signal; v

. .lnarad e uenc syst m he, om ination of means f or generating two radidfrc'quency carrier signals of different frequencies, an electrical output circuit, a pair of electronic tubes connected to be responsive to said respective carrier signals and to supply signals of said different frequencies to said electrical output circuit, multivibrator means connected to supply blocking signals alternately to said tubes so that alternate carrier pulses of said different frequencies are fed to said electrical output circuit, and means for applying a modulating signal to said multivbrator means to cause the widths of the pulses therefrom to vary in accordance with the amplitude of said modulating signal while maintaining the total width of each consecutive pair of pulses substantially constant.

12. In a radio frequency system, the combination of means for generating two radio frequency carrier signals of different frequencies, an electrical circuit resonant at one of said frequencies, electrical means for changing the electrical constants in said electrical circuit to render it resonant at the other of said frequencies, elec-- trical control means for rendering said electrical means alternately effective and ineffective to change the electrical constants in said electrical circuit, thereby rendering said electrical circuit resonant alternately at said different frequencies and producing alternate carrier pulses of said different frequencies, and means for supplying a modulating signal to said electrical control means to effect modulation of the widths of said carrier pulses in accordance with the magnitude of said modulating signal while maintaining the total pulse width for each consecutive pair of carrier pulses substantially constant.

13. In a radio frequency system, the combination of means for alternately producing radio frequency carrier pulses of different frequencies, means for modifying the widths of said carrier pulses in accordance with a modulating signal, while maintaining a susbtantially constant pulse frequency, and means for modifying the pulse frequency in accordance with a second modulating signal, while maintaining a substantially con stant pulse time proportion between each consecutive pair of pulses.

14. In a radio frequency system, the combination of means for generating two radio frequency carrier signals of different frequencies, an electrical output circuit, a pair of electronic tubes connected to be responsive to said respective carrier signals and to supply signals of said different frequencies to said electrical output circuit, multivibrator means connected to supply blocking signals alternately to said tubes so that alternate carrier pulses of said different frequencies are fed to said electrical output circuit, means for applying a modulating signal to said multivibrator means to cause the widths of the pulses therefrom to vary in accordance with the amplitude of said modulating signal while maintaining the total width of each consecutive pair of pulses 11 12 substantially constant, and means for modulatducting means to render it nonconducting and ing the pulse rate of said multivibrator means in to disconnect said element from the, systemz-,; 1 accordance with a second modulating signal. CLARENCE H. McSHAN.

15. In an electrical system, the combination of Y 7 I V 7 an auxiliary electrical element connected in said 5 REFERENCES E F y em, ni-dir t nal c nductin means Thefollowing references are of record in the nected in series with said element, and means for file of this patent: supplying one signal to said uni-directional conducting element to render it conducting and to UNITED STATES PATENTS connect said element to the system and for sup- 10 Number Name Date plying another signal to said uni-directional con- 2,425,066 Labin 'et a1. Aug. 5, 1947 Disclaimer 2,451,347.Olarence H. McShan, Newark, N.

MODULATION. Patent dated Oct. 12, 1

by the inventor. Hereby enters this disclaimer to claim 1 of said patent.

r [Oflicial Gazette August 2, 1949.]

J. FREQUENCY SHIFT PULSE TIME 948. Disclaimer filed July 1, 1949,

Certificate of Correction October 12, 1948.

Patent No. 2,451,347.

CLARENCE H. MoSHAN It is hereby certified that errors appear in the printe numbered patent requiring correction as follows:

Column 1, line 4, for the word modulation read modulating; line 11, for reaction read reacta'nce; co umn 7, line 43, after regular insert or; and that the said Letters Patent should be read with these corrections therein that he record of the case in the Patent Oflice.

the same may conform to t Signed and sealed this 18th day of January, A. D. 1949.

d specification of the above THOMAS F. MURPHY,

Assistant Omismmr of Patnts. 

